Pigs are currently available in many designs for many operations for diverse industries. Pigs can be passively flowing with the flow in a pipeline, without external connection, or the pig can be connected to, for example a line, a pipe or coiled tubing. Such a connection is usually used for withdrawing the pig. A description of pig technology can be found in U.S. Pat. No. 6,651,744 B1 entitled “Bi-Directional Thruster Pig Apparatus and Method of Utilizing Same” which is considered the closest prior art to the present invention. Relevant background art is also described in U.S. Pat. No. 6,315,498 B1 entitled “Thruster Pig Apparatus For Injecting Tubing Down Pipelines”; U.S. Pat. No. 6,122,791 entitled “Retrievable Pig”; U.S. Pat. No. 6,343,657 B1 entitled “Method of Injecting Tubing Down Pipelines”; and U.S. Pat. No. 6,260,617 B1 entitled “Skate Apparatus for Injecting Tubing Down Pipelines”. The above-identified patents are incorporated herein by reference.
A thruster pig is urged into a pipe by pumping a fluid into the pipe at a location behind the pig such that the pig is pushed further into the pipe by said pressurized fluid. If the pipe bore is completely blocked with, for example, hydrates, only a thruster pig system with a return flow line or flow way is useful, since fluid ahead of the pig must be taken out to avoid pressure buildup against the blockage. Thruster pigs having a separate flow line connected have so far been designed with connections and flow line both for fluid flow delivery or return, as well as for withdrawing both the coiled tubing and the connected pig. The separate flow line often consists of coiled tubing.
The insertion and withdrawal of pigs connected to coiled tubing become more difficult as the injection length of the pig and coiled tubing increases, the number of bends increases, and the extent of horizontal or upwards sloping sections in the pipe increases. Such difficulties limit the usability of thruster pigs.
Particularly in the oil and gas industry the demand for pigging operations is significant. Severe problems often occur in long subsea pipelines at large depths and in cold waters. Such problems may include hydrate formation blocking the pipelines, in addition to various deposits such as ice, wax and debris. The initially warm well fluid is cooled down by cold seawater, thereby inducing condensation, precipitation and hydrate formation. Hydrates can be melted with thermodynamic inhibitors such as MEG (glycol coolant) or methanol shifting the melting point of the hydrates to below the ambient temperature. In case of a shutdown or failure in the inhibitor injection system, severe blocking by hydrates is likely.